mm: update _mapcount and page_type documentation

Patch series "mm: page_type, zsmalloc and page_mapcount_reset()", v2.

Wanting to remove the remaining abuser of _mapcount/page_type along with
page_mapcount_reset(), I stumbled over zsmalloc, which is yet to be
converted away from "struct page" [1].

Unfortunately, we cannot stop using the page_type field in zsmalloc code
completely for its own purposes.  All other fields in "struct page" are
used one way or the other.  Could we simply store a 2-byte offset value at
the beginning of each page?  Likely, but that will require a bit more
work; and once we have memdesc we might want to move the offset in there
(struct zsalloc?) again.

...  but we can limit the abuse to 16 bit, glue it to a page type that
must be set, and document it.  page_has_type() will always successfully
indicate such zsmalloc pages, and such zsmalloc pages only.

We lose zsmalloc support for PAGE_SIZE > 64KB, which should be tolerable. 
We could use more bits from the page type, but 16 bit sounds like a good
idea for now.

So clarify the _mapcount/page_type documentation, use a proper page_type
for zsmalloc, and remove page_mapcount_reset().

[1] https://lore.kernel.org/all/20231130101242.2590384-1-42.hyeyoo@gmail.com/


This patch (of 6):

Let's make it clearer that _mapcount must no longer be used for own
purposes, and how _mapcount and page_type behaves nowadays (also in the
context of hugetlb folios, which are typed folios that will be mapped to
user space).

Move the documentation regarding "-1" over from page_mapcount_reset(),
which we will remove next.  Move "page_type" before "mapcount", to make it
clearer what typed folios are.

Link: https://lkml.kernel.org/r/20240529111904.2069608-1-david@redhat.com
Link: https://lkml.kernel.org/r/20240529111904.2069608-2-david@redhat.com
Signed-off-by: David Hildenbrand <david@redhat.com>
Tested-by: Sergey Senozhatsky <senozhatsky@chromium.org>	[zram/zsmalloc workloads]
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Cc: Matthew Wilcox (Oracle) <willy@infradead.org>
Cc: Mike Rapoport (IBM) <rppt@kernel.org>
Cc: Minchan Kim <minchan@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

1 files changed, 0 insertions, 5 deletions

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 51fe207026f8..61a7f680f0ba 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -1210,11 +1210,6 @@ static inline int folio_entire_mapcount(const struct folio *folio)
 	return atomic_read(&folio->_entire_mapcount) + 1;
 }
 
-/*
- * The atomic page->_mapcount, starts from -1: so that transitions
- * both from it and to it can be tracked, using atomic_inc_and_test
- * and atomic_add_negative(-1).
- */
 static inline void page_mapcount_reset(struct page *page)
 {
 	atomic_set(&(page)->_mapcount, -1);